Orthodontic appliances are often attached to a labial or lingual tooth surface by using a bonding material such as cement which is rigid upon setting Typically, in one step of the attachment process, the appliance base is coated with bonding material and then the base is pressed against a tooth surface. It is desirable, of course, that the resulting bond be strong enough to withstand forces exerted on the appliance during treatment. However, as is well known, failure of any device first occurs at the weakest point, and in the application of orthodontic devices to the surface of a tooth, the weakest point will be in the bond by design. As is well known, orthodontic adhesives are specifically tailored to provide this weakest point for the controlled debonding at the end of treatment. However, the problem typically is to select an adhesive having adequate strength for treatment that will still allow removal of the bracket at the end of treatment without causing damage to the tooth.
While there are many forces that act on the appliance, the resulting tensile/compressive forces are of interest herein. Due to the curvature, both mesio-distally and occlusal-gingivally, of tooth surfaces, many prior appliances have been susceptible to stress on the bond from tensile/compressive forces which may ultimately cause the bond to break resulting in appliance detachment. For example, a compressive force along one edge of the appliance will be transmitted directly to the bond. If there is a pivot point between the opposite edges of the appliance due to tooth curvature or other surface incongruities, the compressive force on the one edge may cause the tensile force on the bond near the opposite edge to be so great as to detach the appliance. Therefore, any place where support to resist a force is provided by the bond rather than part of the base, a failure is more likely to occur. In order to reattach the appliance, an unnecessary visit to the orthodontist is required and treatment is interrupted.
A specific example of this problem with tensile/compressive forces is illustrated by the tendency of an appliance to detach when a patient bites into a resistive food and masticatory forces are applied to the appliance. The masticatory forces tend to compress the bond near one edge of the base and tension the bond near the other edge. The resulting shear forces on the bond may cause the appliance to peel away from the tooth surface beginning at the edge in tension.
Another problem with known mounting bases is a tendency for air pockets to remain in the bonding material after application. Many known appliance bases are fabricated to include holes or recesses and/or for use with meshes in attempts to increase the bonding surface area of the appliance. However, it is difficult to apply the bonding material to expel air pockets adjacent to such surfaces in many appliances. For example, when pressed against a tooth surface, these base designs do not accommodate air pocket evacuation responsive to the compressive forces communicated through the bonding material due to a lack of open channels for the air and the bonding material to flow.
Orthodontists are typically concerned about the "seating" of the appliance on the tooth. Thus there is a tendency to give the appliance one more push after positioning the appliance on the tooth with adhesive after the adhesive has started to "set" or polymerize. This tendency may cause the base to "rock" and create a weaker bond. Any rocking after polymerization of the adhesive has begun is likely to be detrimental to the overall strength and consistency of the bond.
Additionally, when using ceramic appliances there is a problem with breakage during removal of the appliance after treatment. Any broken parts are possible hazards to the patient and must be carefully removed whether loose in the mouth or stuck on the tooth. Any portions remaining on the tooth surface are especially troublesome as they must be ground off with a diamond burr which may result in damage to the surface of the tooth.
Thus, there is a need for an improved orthodontic appliance base which is stable on tooth surfaces of various curvatures thereby reducing the likelihood of separation therefrom and the resulting appliance failure. In addition, there is a need for an orthodontic appliance base which provides greater resistance to masticatory forces. There is also a need for an orthodontic appliance base which provides a large bonding surface area while reducing the likelihood that air pockets will remain between the bonding material and the base upon application to a tooth.